1. Field of the Invention:
The present invention relates to a system for initializing transmitters used in high capacity digital radio links, in which multilevel QAM digital signals are sent over a carrier wave. The initialization is carried out by generating a plurality of sinusoidal signals (tones), adding these tones together to simulate a digital signal to be transmitted, sending the simulated digital signal through the transmitter, and adjusting the transmitter to compensate for the intermodulation odd order products developed as a result of the nonlinearity of the power amplifier in the transmitter.
2. Description of the Related Art:
Transmitter linearity is an important quality parameter in high capacity digital radio links that utilize multilevel quadrature amplitude modulation, particularly in the case where there are high number of modulation levels (e.g. 64 or 256 QAM).
The power amplifiers used in transmitters are intrinsically subject to distortion caused by nonlinearity of both even order and odd order. These latter nonlinearities are particularly dangerous because they cause intermodulation products that fall inside the transmitted digital signal band and therefore they cannot be eliminated by simple filtering.
To compensate for the effect of these amplifier nonlinearities of odd order and in particular of the third order, an I.F. (intermediate frequency) predistortion technique is often used. It generates an amplitude and phase distortion complementary to the distortion caused by the power amplifier; thus the distortion is compensated and annulled. Alignment is obtained by amplitude and phase adjustments in the predistortion circuit.
FIG. 1 shows an overall scheme of the transmitter: Predistorter 1 represents an IF linearizer formed by a predistorter for distortion compensation; R.sub.1 and R.sub.2 indicate the amplitude and phase adjustments of the distortion introduced by the predistorter; OL is the carrier wave generated by a local oscillator that is applied to the converter 2 to convert the IF signal of the predistorter to the transmission frequency; the power amplifier is indicated by reference numeral 3. The criterion by which the above-indicated compensation is optimized represents a very critical feature. This subject has not yet been considered by authorities generally recognized and accepted in the world.
Among the most common criteria used is the minimization of intermodulation products measured at the output of the power amplifier. FIG. 2 shows the related measure bench, in which the test signal generator 4 generates the test signal that is sent into predistorter 1, and converter 2, and power amplifier 3; FIG. 2 also shows the spectrum analyzer 5, which displays the behavior of the amplifier test signal after it has been subjected to the intermodulation distortion of the power amplifier PA.
Using this procedure, the greatest problem to solve is the selection of the test signal SP to use for optimizing the system performance. This signal must have characteristics similar to the characteristics of the digital signal. In particular, it must have analogous bandwidths, mean power and amplitude dynamics. This last parameter is very important, but up to now it hasn't been considered with sufficient attention.
A QAM signal exhibits a ratio Pp/Pm (Pp=peak power; Pm=mean power) that depends on:
modulation levels number PA0 filtering type PA0 roll-off factor PA0 modulation 64 QAM PA0 raised cosine filtering PA0 roll-off factor=0.35 PA0 (Pp/Pm)dB=8 dB
As a result of overelongations that occur in the transitions from one status to another status of the constellation that represents the geometrical locus of transmitted symbols (see FIG. 3), the instantaneous values of power obtained may be considerably greater than the mean power value.
For example, in the case of:
the result is:
A quality test of the relationship that typically exists between input power and output power in a power amplifier (see FIG. 4) indicates that the distortion in proximity to the peak value Pp differs from the distortion corresponding to lower power values. That is to say, the spectrum of the output signal and, in particular, the amplitude of intermodulation products depends on the amplitude dynamics of the same signal. Therefore, the test signal must cover the characteristics of the power amplifier 3 with an instantaneous power distributed in the same manner as the digital signal. In particular, it must have the same ratio Pp/Pm.
test signals presently used are:
1) DIGITAL SIGNAL:
Just the signal that must be transmitted. The spectrum analyzer 5 displays lateral lobes due to intermodulation of amplifier 3. Therefore, the compensation of such intermodulation is made minimizing the power associated to these lobes. As a result:
the amplifier 3 is charged in the actual operating condition; PA1 it is difficult to find the desired minimum condition (just by looking at spectrum analyzer display); and PA1 the performances are generally unsatisfactory. PA1 Pp/Pm increases with the number N of tones: PA1 the distribution of the amplitudes is uncontrollable (the time percentage during which the instantaneous amplitude approximates the peak value Pp decreases when N increases); PA1 the performances for modulation with high level number are unsatisfactory. PA1 Pp/Pm=3 dB; PA1 performances are poor for modulation at high level number. PA1 Pp/Pm=6 dB; PA1 performances are optimal for 16 QAM modulations; PA1 performances for modulations with greater number of levels are unsatisfactory. PA1 good performances for modulations with high level number; PA1 high fulfillment complexity (all transmission systems are running in automatic level control, thus keeping the mean power of transmitted signal constant vs. time). The use of such a test signal requires that the transmitter must be put in manual control, be adjusted to get the minimization of intermodulation products and then be put in operation with automatic control. Therefore, this procedure is complex and not practical.
2) MULTITONE SIGNALS with means power equal to mean power of digital signal.
These are obtained by summing N sinusoidal signals at different frequencies, with the result that:
2a) 2 TONES with mean power equal to mean power of digital signal.
These 2 tones are obtained by summing two sinusoidal signals (tones) at different frequency, with the result that:
2b) 4 TONES with mean power equal to mean power of digital signal. These 4 tones are obtained by summing four sinusoidal signals at different frequencies, and the following characteristics occur:
2c) 4 TONES with mean power that differs from the mean power of digital signal by [(Pp/Pm)dB-6]dB, where Pp is the peak power and Pm is the mean power of digital signal.
For this signal, the summing of four tones of different frequency results in: